Canon EOS 5D Mark III Instruction Manual (en)

 Multiple ExposuresN
Shoot subsequent exposures.
When [On:Func/Ctrl] is set, the merged multiple-exposure 
image will be displayed.
With Live View shooting, the multiple-exposure images merged 
so far will be displayed. By pressing the <
B> button, you can 
display only the Live View image.
After you shoot the set number of exposures, multiple-exposure 
shooting will be canceled. With continuous shooting, after you 
finish shooting the set number of exposures while holding down 
the shutter button, the shooting will stop.
With multiple exposures, the more exposures there are, the more noticeable 
the noise, irregular colors, and banding will be. Also, as noise increases with 
higher ISO speeds, shooting at low ISO speeds is recommended.
If [Additive] is set, the image processing after taking the multiple exposures 
will take time. (The access lamp will light up for longer than usual.)
If you perform Live View shooting while [On:Func/Ctrl] and [Additive] are 
both set, the Live View function will stop automatically when the multiple 
exposure shooting ends.
In step 9, the brightness and noise of the multiple-exposure image 
displayed during Live View shooting will be different from the final multiple-
exposure image recorded.
If [On:ContShtng] is set, let go of the shutter button after shooting the set 
number of exposures.
If the power switch is set to <2> or the battery is replaced after you set 
multiple exposure settings, multiple-exposure shooting will be canceled.
If you switch the shooting mode to <
A> <w/x/y> while shooting, 
multiple-exposure shooting will end.
When multiple exposure is set or during multiple-exposure shooting, you 
cannot use the functions dimmed in the camera menu.
If you connect the camera to a personal computer or printer, multiple-
exposure shooting is not possible.
When [On:Func/ctrl] is set, you can press the <x> button to view the 
multiple exposures taken so far or delete the last single exposure (p.184).
You can also set multiple exposure with [z3: Multiple exposure].